The invention relates to a soft magnetic multilayer film comprising a number of crystalline main layers and a number of amorphous secondary layers, the layers being alternately provided and the magnetic constituent of the main layers being substantially exclusively Fe. The invention also relates to a magnetic head which is provided with such a soft magnetic multilayer film.
A soft magnetic multilayer film of this type is known from Abstract No. F1-04 on page 366 of the Book of Abstracts of the ICMFS-12 conference, which was celebrated in Le Creusot in 1988. This literature reference more particularly describes a multilayer film having main layers of crystalline Fe and secondary layers of amorphous CoNbZr. These layers are alternately provided by means of sputtering. The amorphous secondary layers serve to limit the grain size of the crystallites of the main layer. The well-known multilayer film exhibits a number of suitable soft magnetic properties such as a high saturation magnetisation (B.sub.s), a small coercive force (H.sub.c) and a high magnetic permeability (.mu.). For example, the well-known multilayer film, which has an overall thickness of 0.5 .mu.m and which consists of alternately applied layers of 10 nm thick crystalline Fe and 5 nm thick amorphous Co.sub.86.2 Nb.sub.9.6 Zr.sub.4.2, has a saturation magnetisation of 1.8 T, a coercive force of 60 A/m and a permeability of at least 2000 in the frequency range up to 20 MHz.
Applicants have found that the well-known soft magnetic multilayer film has the disadvantage that it has a large magnetostriction constant (.lambda..sub.s). The absolute value of this constant even considerably exceeds the individual values of crystalline Fe and amorphous CoNbZr. It was found, for example, that a multilayer film composed of 40 layers of 5 nm thick amorphous Co.sub.86.2 Nb.sub.9.6 Zr.sub.4.2 and 40 layers of 10 nm thick crystalline Fe, both types of layers being alternately applied, has a magnetostriction constant of +10.times.10.sup.-6. The magnetostriction constants of crystalline Fe and amorphous CoNbZr are -5.times.10.sup.-6 and +0.2.times.10.sup.-6, respectively. On the basis of the two last-mentioned values and the thicknesses of the individual layers a multilayer magnetostriction constant of -2.4.times.10.sup.-6 may be expected. Consequently, the actual magnetostriction considerably exceeds the expected value. If the known multilayer film is used in a magnetic head the high value of the magnetostriction constant forms a serious problem because an increase of the magnetostriction constant brings about a decrease of the signal/noise ratio (SNR-ratio). Consequently, the aim in the manufacture of multilayer film for use in magnetic heads is to minimize the absolute value of the magnetostriction constant of the film, preferably below 5.times.10.sup.-6.
The well-known multilayer film has the additional disadvantage that the value of the magnetostriction constant was found to have increased considerably after a heat treatment. For example, after exposing the film to a firing treatment of 300.degree. C. for one hour it was found that the magnetostriction constant of the multilayer film mentioned in the preceding paragraph had more than tripled to a value of +30.times.10.sup.-6. This is problematic, in particular, when soft magnetic multilayer films are used in magnetic heads, because at present such a film is customarily subjected to heat treatments in the magnetic head.